An ion implantation process typically requires a uniform and consistent dose or amount of ions to be implanted into a wafer. Ion implantation processes also typically require a stable and uniform ion beam for implanting into the wafer. The dose is generally a function of ion beam current density and time that the wafer spends in front of an ion beam.
Because of larger and larger wafer sizes, recent semiconductor manufactures have moved towards processing one wafer at a time, in so-called single-wafer systems. In general, a so-called ribbon ion beam is employed by the single-wafer ion implantation. Clearly, when the required length of the ribbon beam is continually increased for larger and larger wafer sizes, it becomes more difficult to set up a ribbon ion beam meeting the required levels of uniformity in at least both beam intensities and angles. Therefore, there is a need for an improved method and apparatus capable of achieving uniform implantation in a single-wafer ion implanter without requiring extensive beam uniformity tunes.
A conventional technique entails simultaneously moving and rotating a wafer, which is larger than an ion beam, such that different portions of the wafer receive a substantially uniform dose. First, as shown in FIG. 1A, an ion beam 10 is formed with an elongated shape having a length and a width along two independent axes of the ion beam 10, and a wafer 11 is provided with a diameter larger than both the length and the width of the ion beam 10. Second, as shown in FIG. 1B, the wafer 11 is simultaneously rotated and moved across the ion beam 10. Clearly, the rotation of the wafer 11 can be used to ensure the wafer 11 is completely implanted by the ion beam 10, as the ion beam 10 is shorter than the diameter of the wafer 11. Moreover, as shown in FIG. 1C, different points on the wafer may have different rotation trajectories when a projection 12 of the ion beam 10 onto the wafer 11 is moved a unit distance (Δd), as can be discerned with reference to points A, B and C. Significantly, as shown in FIG. 1C, different points may have very different implantations during the different rotation trajectories. Therefore, to ensure that different points on the wafer have essentially equivalent implantation results, it is necessary for each point to be rotated around a center of the wafer at least one time, even integral times, during the unit distance Δd. In other words, the rotation velocity must be significantly higher than the movement velocity. Moreover, as shown in FIG. 1D, by comparison of how the rotary trajectories of points D, E and F are projected by the ion beam 10, the projected ratio of a corresponding rotation trajectory of a point is decreased as the distance between the point and the center of the wafer is increased. Therefore, to achieve subsequently equivalent implantations, the point having lower (higher) projection ratio should be projected for a longer (shorter) period of time. In other words, the movement velocity of the wafer 11 across the ion beam 10 must be higher when the ion beam is near the center of the wafer 11, and lower when the ion beam is near the edge of the wafer 11, as shown in FIG. 1E.
Another conventional technique is to scan an ion beam in multiple rotationally-fixed orientations of a wafer at movement velocities, such that different portions of the wafer receive substantially uniform doses. First, as shown in FIG. 2A, an ion beam 20 is formed as an elongated shape having a length and a width along two independent axes, and a wafer 21 is provided having a diameter with no relation to the length and the width. To emphasize the differences between this embodiment and the previous embodiment, the corresponding figures show the case where the diameter is larger than both the length and the width. Second, as shown in FIG. 2B, the wafer 21 is located on the left side of the ion beam 20 and has at least some separate points A, B, C and D. Third, as shown in FIG. 2C, the wafer 21 is moved through the ion beam 20 to the right side of ion beam 20. The wafer 21 is not rotated during the movement of the wafer 21. Fourth, as shown in FIG. 2D, the wafer 21 is rotated, such that the relative geometric relations among the ion beam 20 and the points A, B, C, D are changed. Fifth, as shown in FIG. 2E, the ion beam 20 is moved through the ion beam 20 to the left side of the ion beam 20, when the wafer 21 is not rotated during the movement of the wafer 21. After that, the above “movement-rotation-movement-rotation” process is repeated, until the wafer 21 is essentially uniformly implanted. When the number of repeated times is sufficient, the accumulated implantations on each of points A-D during the multiple rotationally-fixed orientations will essentially be equivalent. In other words, the final implantation result is independent of where the points A-D are in the beginning as shown in FIG. 2B.
However, these conventional technologies still have some non-negligible disadvantages. For the former conventional technology, the required rapid rotation may damage the wafer by a couple of different ways. For example, the fine scale structures formed on the wafer may not have sufficient structural integrity to withstand the centripetal acceleration, and the rotation can greatly add to the kinetic energy when particles collide with the wafer surface thus enhancing the destructive potential of the particles. Moreover, the mechanism for simultaneously moving and rotating the wafer is more complex than separately moving and rotating the wafer, especially when the rotation velocity is high. For the later conventional technology, the proper number of repetitions can be less than clear. Here, with more repetitions, implantation uniformity is increased but at the expense of decreased throughput. Clearly, the throughput will be significantly decreased when the ion beam is shorter than the diameter of the wafer or the stable portion of the ion beam is shorter than the diameter of the wafer, because many repeated times will be needed or desired to ensure different portions of the wafer are uniformly implanted.
Accordingly, there remains no ideal technology for uniformly implanting a wafer using a ribbon ion beam. A need thus exists to develop such a new technology for achieving this issue, especially to develop a new technology for effectively achieving this issue without significantly having to amend conventional technologies.